A brushless direct current (BLDC) motor has a stator and a rotor. The stator has a stator core with a winding wound thereon. An electric current flowing through the winding generates a magnetic field. An electromagnetic force is generated by the energized conductor cutting magnetic flux lines, which causes the rotor to rotate with respect to the stator, whereby the electrical energy is converted into kinetic energy. The stator core is usually formed by a stack of core laminations. As the BLDC motor is becoming widely used, there is a demand for the motor to have higher performance including smaller size and greater output power. The size of the stator is mainly determined by the size of the stator core.
As shown in FIG. 1, an electric motor 10 with a conventional stator core 11 has a generally round annular shape, which has an inner circumferential surface with a plurality of teeth 13 formed thereon. A winding slot 12 is formed between adjacent teeth, and all the winding slots 12 have the same shape. While a stator core with such a configuration is easy to fabricate, it is difficult to further increase its output power, due to a lack of space to increase the size of the winding slots.